1. Field of the Invention
The invention relates to sensing chemical, biochemical and physical measurands using piezoelectric sensors.
2. Description of the Related Art
There are a number of sensor applications that demand exceptional physically, chemically and environmentally stable sensing elements. Two classes of sensors which are frequently proposed to this end are the optical fiber based sensors and the piezoelectric sensors, both of which derive their chemical and physical stability from solid state construction of inert glassy or crystalline solids. One advantage of the piezoelectric sensors is their wide range of potential sensing mechanisms coupled with the exceptional temperature stability of the most common implementation, namely quartz crystal technology.
Various piezoelectric sensor geometries have been proposed and patented. The most widely considered of these is the quartz crystal microbalance, though other piezoelectric crystals, polymers and composites have also been used. Though the original patents in this field are expired, the technology is still not a mature technique for many sensing applications.
U.S. Pat. No. 4,760,351, issued to Newell et al., teaches the use of arrays of these devices which consist of a parallel plate capacitor employing a piezoelectric material as the dielectric support. While this structure has exhibited good sensing characteristics in the vapor phase, sensor performance is substantially impaired when fluid phase operation is pursued.
U.S. Pat. No. 5,374,521, issued to Kipling et al., discloses an alternative mode of operation of these sensors intended to overcome these difficulties; however, the method is not amenable to field deployment or mass production.
The preferred methods of operating the crystal sensor are (1) using an impedance analyzer in a laboratory setting or (2) incorporating the device into an oscillating circuit. Under vapor phase operation, both techniques are suitable; however, liquid phase operation incurs many difficulties in instrumentation, which substantially impair the reliability and sensitivity of the sensor.
U.S. Pat. No. 4,847,193, issued to Richards et al., discloses a signal amplification technique which partially overcomes this problem in selected assays. U.S. Pat. No. 5,179,028, issued to Vali et al. discloses an alternative structure which replaces the parallel plate capacitor with a tuning fork geometry. This geometry appears to offer other measurement methods but does not overcome the difficulties associated with liquid phase operation.
U.S. Pat. No. 4,735,906, issued to Bastiaans, discloses a surface wave device which supports two separate interdigital transducers which serve to convert energy between electrical and acoustic signals. These transducers employ arrays of electrodes which are periodically spaced on the surface of the crystal and are alternately connected to the positive and negative terminals of the electrical input or output.
U.S. Pat. No. 5,306,644, issued to Myerholtz et al. teaches an improvement on Bastiaans invention by employing a specific combination of materials and structural design to substantially reduce mechanical interactions with the fluid medium. Myerholtz et al. also discloses the expansion of a single sensor to an array.
U.S. Pat. No. 5,478,756, issued to Gizeli et al. teaches an alternative improvement on the Bastiaans structure which employs a thick film of dielectric material to help confine the acoustic signal to the surface.
All these approaches suffer from two significant limitations. The foremost of these is that the structures do not isolate the electrical connections from the sensing environment. One solution has been to employ an acoustic waveguide version of the Bastiaans structure. While this structure overcomes the most substantial limitations of the Bastiaans structure, it incurs additional spurious resonances which further complicate the sensor instrumentation. Finally, all of the Bastiaans-derived structures require micron scale lithography and teach towards complicated, surface-based devices. A simpler implementation is based upon the (quartz) crystal microbalance devices. The fundamental problem with these techniques, however, is that the sensing element is a simple reactive electrical component with a single electrical "port".
There is not found in the prior art a sensing apparatus which has no additional spurious resonances, separate input and output terminals, and isolates the electrical connections to the circuit from the sensing environment.